During the period between the Great European War (1914–18) and the Pacific War (1941–45) the Japanese expansionist impulse in the South Seas expressed itself through emigration and economic enterprise abroad. There were Japanese settlements in almost every country in the region. The larger ones were in the Philippines, Malaya, and the Netherlands East Indies. On the eve of the Pacific War the estimated number of Japanese residents in the South Seas was 24,000, with investments totalling around ¥250 million. Compared to those of the Western colonial powers and the immigrant Chinese, the size of these investments was insignificant and their numbers meagre. But this only served to spur those interested in Japan's economic expansion in the South Seas to make greater efforts to achieve their aims. External events helped to realize these objectives. For a brief interregnum during the Japanese Occupation (1942–45) the Japanese became the politically dominant community in the region with control over its economic resources.

Studies of Indonesia in the twentieth century have properly concentrated on the history of the nationalist movement, but its external relations have been unduly neglected. It is indeed a commonplace that the Japanese invasion had a decisive impact on the fortunes of the nationalist movement and of its antagonists. But the invasion was the climax of a sequence of developments that brought the security of the Netherlands' tenure of Netherlands India into question. Apart from their intrinsic interest, those developments provide an additional context within which to appraise the nationalist movement and the Dutch reaction to it.

This paper derives from a larger study of the nature of Japan's relations with colonial South and Southeast Asia in the period between the Russo-Japanese War and the Pacific War. By means of a detailed examination of a single facet of Japanese-Philippine relations, it is hoped that a greater insight may be gained into the often convolute processes of the interactions between, on the one hand, the dominant Asian power of the inter-war period, and on the other hand, colonial entities and personalities still beholden to Western European or North American rulers. However, two caveats need to be put forward about this essay: (1) the case of the Philippines was unique in colonial Asia since the United States had fully committed itself to a policy of withdrawal, thus facilitating contacts between the local ruling elite and Japanese diplomats; (2) despite pre-war and wartime propaganda to the contrary, the principal concern of Japan in all its dealings with colonial South and Southeast Asia before the Pacific War was economic. In the prior instance, therefore, the paragraphs that follow will demonstrate an apparently remarkable degree of freedom of action on the part of the Filipinos in authority under the Commonwealth Administration (1935–46) in spite of the continuing legal responsibility of the United States for Philippine foreign affairs under the provisions of the Tydings-McDuffie (Independence) Act of 1934. The second caveat will be evidenced by the unstinting and continuous attention of Japanese diplomats to the development of ever closer economic ties between the Philippines and Japan.

During the Second World War, there was a momentous drive to build a new Thai society under the Government of Pibulsongkram, popularly known as Pibul. The wave of these changes lasted only about five years, but it had an enormous effect on all aspects of Thai life.

Military consideration had been given top priority by the Japanese policy-makers in formulating policy towards Burma immediately prior to and after the outbreak of the Pacific War. This was evident in their decision to establish military administration in Burma in March 1942, and in granting independence to the Burmese in August 1943. These decisions, however, had an adverse effect for Japan during the war years, since they were largely responsible for turning the Burmese against Japan.

For the greater part of the period from 1940 to 1945, French Indochina occupied a peculiar place in the Co-Prosperity Sphere of Great East Asia. Following the capitulation of France before the German army in June 1940, the northern part of Indochina became the first area in Southeast Asia to admit Japanese troops — at that stage, as an extension of the Japanese campaign in southern China. In July 1941 the Japanese advance into southern Indochina marked the first step towards a full-scale attack on European and American possessions in the whole region, which materialized in December 1941. But this military advance into Indochina, precisely because it took place before the general assault on Southeast Asia and the Pacific, had to be achieved by means of agreements and treaties with an established government. These were possible only because the French in Indochina decided to recognize the pro-German government at Vichy, so that the Japanese were able to apply diplomatic pressure both in France and at Hanoi. Once the agreements had been made, the Japanese saw no need to change the basis of their occupation of Indochina even after December 1941; they were by then preoccupied with establishing their presence in other areas. Consequently they continued to recognize the French administration in Indochina and to maintain diplomatic relations with it, so long as diplomatic pressure was sufficient to ensure that Japanese military needs were fully met.